1. Field of the Invention
The present invention relates to a field effect semiconductor device, and more particularly to a field effect group III-V compound semiconductor device with a Schottky gate electrode.
2. Description of the Related Art
A group III-V compound semiconductor has a carrier mobility higher than Si or Ge semiconductor. Two dimensional electron gas having a very high mobility can be obtained by using a narrow potential valley at a hereto junction and a group III-V compound semiconductor crystal having a low impurity concentration.
For example, a high electron mobility transistor (HEMT) is known as a Field effect semiconductor device using such a two dimensional electron gas. A high speed compound semiconductor device such as HEMT can be used for high speed computers or other devices.
HEMT has been realized initially by using an AlGaAs/GaAs hereto junction. It has been found thereafter that AlGaAs contains DX centers of a deep level trap which can cause a distortion in an I-V characteristic curve. Another type of HEMT has been developed which incorporates an InGaP/GaAs hereto junction not containing DX centers. For a channel (electron transfer or electron travelling) layer, InGaAs is also used as a substitute for GaAs.
A good insulating film such as a silicon oxide film on a Si crystal has not been found as yet for a group III-V compound semiconductor crystal. Therefore, an insulating gate electrode such as a MOS type control electrode of an Si semiconductor device cannot be used For group III-V compound semiconductor crystal, but a Schottky electrode can be used for its gate electrode. A Schottky contact is Formed by a metal/semiconductor contact so that a forward current can flow if a forward bias equal to a Schottky voltage or higher is applied.
FIG. 6 shows an example of the structure of a HEMT manufactured by conventional techniques. On the surface of a semi-insulating GaAs substrate 21 doped with Cr or another element, a non-doped GaAs buffer layer 22 is epitaxially grown, and thereafter a non-doped GaAs active layer 23 is epitaxially grown on the buffer layer 22. On the active layer 23, a non-doped AlGaAs spacer layer 24, an Si-doped n-type AlGaAs electron supply layer 25, and an Si-doped n-type GaAs contact layer 27 are epitaxially grown in this order. The contact layer 27 is partially removed to form thereon a Schottky gate electrode 29 made of Al, Wsi.sub.x, TiW or another clement. On the contact layer 27, source/drain electrodes 28 are formed and ohmic contacts are formed by alloying AuGe/Au, Ni/AuGe/Au and so on.
AlGaAs contains DX centers which adversely affect the characteristics of a HEMT. The electron supply layer 25 may be made of InGaP instead of AlGaAs. In this case, the Schottky gate 29 is formed on an n-type InGaP layer.
A MESFET does not use an electron supply layer, and a Schottky gate electrode and a pair of source/drain electrodes on both sides of the gate electrode are formed on an n-type channel layer. A GaAs MESFET has a Schottky gate electrode of Al, WSi.sub.x, or another element Formed on an n-type GaAs layer.
The gate electrode of such a field effect group III-V compound semiconductor device controls the transfer of carriers in the channel layer. A Schottky gate electrode has a Schottky barrier specific to an underlying semiconductor layer. If a forward bias voltage equal to or higher than a Schottky voltage is applied, a forward current will flow through the gate. If the amplitude of an input signal voltage is large and a large forward voltage is applied to the gate electrode, a forward current flows through the gate.
As described above, a conventional Schottky gate electrode of a group III-V compound semiconductor device has a limited value of a Schottky barrier height and a current flows through the gate if a high forward voltage is applied thereto.